The CAS Registry Mumber 81639-11-4 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 8,1,6,3 and 9 respectively; the second part has 2 digits, 1 and 1 respectively. Calculate Digit Verification of CAS Registry Number 81639-11: (7*8)+(6*1)+(5*6)+(4*3)+(3*9)+(2*1)+(1*1)=134 134 % 10 = 4 So 81639-11-4 is a valid CAS Registry Number.